Hi, real world, again!

The Mammal has emerged from a thesis-induced supermassive black hole and a Christmas-induced food coma, only to find that in the month or so that she spent barely functional and buried in chapters covered in the supervisor’s dreaded Red Pen, things actually happened in the world outside. This, naturally, manifested in thousands of items feeling thoroughly neglected in RSS readers and email inboxes. (Jesus. How many times have I vowed never to neglect my RSS feed again? Oh well, it’s not like unemployment is such a busy occupation that I can’t deal with a measly two and a half thousand articles 😛 )

… earlier tonight, the paragraph here said I wasn’t doing a proper post yet, “just pointing out” a couple of the cooler things I’ve missed. Then somehow this thing morphed into a 1000+ word post that goes way beyond “pointing things out”. It’s almost like I’ve been itching to write something that isn’t my thesis. >_>

So the first cool thing I wanted to “point out” is the genome paper of the centipede Strigamia maritima, which is a rather nondescript little beast hiding under rocks on the coasts of Northwest Europe. This is the first sequenced genome of a myriapod – the last great class of arthropods to remain untouched by the genome sequencing craze after many genomes from insects, crustaceans and chelicerates (spiders, mites and co.).  The genome sequence itself has been available for years (yay!), but its “official” paper (Chipman et al., 2014) is just recently out.

Part of the appeal of Strigamia – and myriapods in general – is that they are considered evolutionarily conservative for an arthropod. In some respects, the genome analysis confirms this. Compared to its inferred common ancestor with us, Strigamia has lost fewer genes than insects, for example. Quite a lot of its genes are also linked together similarly to their equivalents in distantly related animals, indicating relatively little rearrangement in the last 600 million years or so. But this otherwise conservative genome also has at least one really unique feature.

Specifically, this centipede – which is blind – has not only lost every bit of DNA coding for known light-sensing proteins, but also all known genes specific to the circadian clock. In other animals, genes like clock and period mutually regulate one another in a way that makes the abundance of each gene product oscillate in a regular manner (this is about the simplest graphical representation I could find…). The clock runs on a roughly daily cycle all by itself, but it’s also connected to external light via the aforementioned light-sensing proteins, so we can constantly adjust our internal rhythms according to real day-night cycles.

There are many blind animals, and many that live underground or otherwise find day and night kind of irrelevant, but even these are often found to have a functioning circadian clock or keep some photoreceptor genes around. However, based on the genome data, our favourite centipede may be the first to have completely lost both. The authors of the genome paper hypothesise that this may be related to the length of evolutionary time the animals have spent without light. Things like mole rats are relatively recent “inventions”. However, the geophilomorph order of centipedes, to which Strigamia belongs, is quite old (its most likely sister group is known from the Carboniferous, so they’re probably at least that ancient). Living geophilomorphs are all blind, so chances are they’ve been that way for the last 300+ million years.

Nonetheless, the authors also note that geophilomorphs are still known to avoid light – the question now is how the hell they do it… And, of course, whether Strigamia has a clock is not known – only that it doesn’t have the clock we’re used to. We also have no idea at this point how old the gene losses actually are, since all the authors know is that one other centipede from a different group has perfectly good clock genes and opsins.

In comparison with fruit flies and other insects, the Strigamia genome also reveals some of the ways in which evolutionary cats can be skinned in multiple ways. There is an immune-related gene family we share with arthropods and other animals, called Dscam. The product of this gene is involved in pathogen recognition among other things, and in flies, Dscam genes are divided into roughly 100 chunks or exons, most of which are are found in clusters of variant copies. When the gene is transcribed, only one of these copies is used from each such cluster, so in practical terms the handful of fruit fly Dscam genes can encode tens of thousands of different proteins, enough to adapt to a lot of different pathogens.

A similar arrangement is seen in the closely related crustaceans, although with fewer potential alternative products. In other groups – the paper uses vertebrates, echinoderms, nematodes and molluscs for comparison – the Dscam family is pretty boring with at most one or two members and none of these duplicated exons and alternative splicing business. However, it looks like insects+crustaceans are not the only arthropods to come up with a lot of DSCAM proteins. Strigamia might also make lots of different ones (“only” hundreds in this case), but it achieved this by having dozens of copies of the whole gene instead of performing crazy editing feats on a small number of genes. Convergent evolution FTW!

Before I paraphrase the entire paper in my squeeful enthusiasm (no, seriously, I’ve not even mentioned the Hox genes, and the convergent evolution of chemoreceptors, and I think it’s best if I shut up now), let’s get to something else that I can’t not “point out” at length: a shiny new vetulicolian, and they say it’s related to sea squirts!

Vetulicolians really deserve a proper discussion, but in lieu of a spare week to read up on their messiness, for now, it’s enough to say that these early Cambrian animals have baffled palaeontologists since day one. Reconstructions of various types look like… a balloon with a fin? Inflated grubs without faces? I don’t know. Drawings below (Stanton F. Fink, Wikipedia) show an assortment of the beasts, plus Yunnanozoon, which may or may not have something to do with them. Here are some photos of their fossils, in case you wondered.

Vetulicolians from Wiki

They’re certainly difficult creatures to make sense of. Since their discovery, they’ve been called both arthropods and chordates, and you can’t get much farther than that with bilaterian animals (they’re kind of like the Nectocaris of old, come to think of it…).

The latest one was dug up from the Emu Bay Shale of Australia, the same place that yielded our first good look at anomalocaridid eyes. Its newest treasure has been named Nesonektris aldridgei by its taxonomic parents (GarcĂ­a-Bellido et al., 2014), and it looks something like this (Diego GarcĂ­a-Bellido’s reconstruction from the paper):

Garcia-Bellido_etal2014-nesonektris_recon

In other words, pretty typical vetulicolian “life but not as we know it”, at first glance. Its main interest lies in the bit labelled “nc” in the specimens shown below (from the same figure):

GarcĂ­a-Bellido_etal2014-nesonektris_notochords

This chunky structure in the animal’s… tail or whatever is a notochord, the authors contend. Now, only one kind of animal has a notochord: a chordate. (Suspicious annelid muscle bundles notwithstanding. Oh yeah, I also wanted to post on Lauri et al. 2014. Oops?) So if this thing in the middle of Nesonektris’s tail is a notochord, then at the very least it is more closely related to chordates than anything else.

Why do they think it is one? Well, there are several long paragraphs devoted to just that, so here goes a summary:

1. It’s probably not the gut. A gut would be the other obvious ID, but it doesn’t fit very well in this case. Structures interpreted as guts in other vetulicolians – which sometimes contain stuff that may be half-digested food – (a) start in the front half of the body, where the mouth is, (b) constrict and expand and coil and generally look much floppier than this, (c) don’t look segmented, (d) sometimes occur alongside these tail rod-like thingies, so probably aren’t the same structure.

2. It positively resembles modern half-decayed notochords. The notochords of living chordates are long stacks of (muscular or fluid-filled) discs, which fall apart into big blocks as the animal decomposes after death. Here’s what remains of the notochord of a lamprey after two months for comparison (from Sansom et al. (2013)):

Sansom_etal2013-adult_lamprey_notochord_d63

This one isn’t as regular as the blockiness in the fossils, I think, but that could just be the vetulicolians not being quite as rotten.

There is, of course, a but(t). To be precise, there are also long paragraphs discussing why the structure might not be a notochord after all. It’s much thicker than anything currently interpreted as such in reasonably clear Cambrian chordates, for one thing. Moreover, it ends right where the animal does, in a little notch that looks like a good old-fashioned arsehole. By the way, the paper notes, vetulicolian tails in general don’t go beyond their anuses by any reasonable interpretation of the anus, and a tail behind the anus is kind of a defining feature of chordates, though this study cites a book from the 1970s claiming that sea squirt larvae have a vestigial bit of proto-gut going all the way to the tip of the tail. (I suspect that claim could use the application of some modern cell labelling techniques, but I’ve not actually seen the book…)

… and there is a phylogenetic analysis, in which, if you interpret vetulicolians as deuterostomes (which impacts how you score their various features), they come out specifically as squirt relatives whether or not you count the notochord. I’m never sure how much stock to put in a phylogenetic analysis based on a few bits of anatomy gleaned from highly contentious fossils, but at least we can say that there are other things – like a hefty cuticle – beyond that notochord-or-not linking vetulicolians to a specific group of chordates.

Having reached the end, I don’t feel like this paper solved anything. Nice fossils either way 🙂

And with that, I’m off. Maybe next time I’ll write something that manages to be about the same thing throughout. I’ve been thinking that I should try to do more posts about broader topics rather than one or two papers (like the ones I wrote about ocean acidification or homology versus developmental genetics), but I’ve yet to see whether I’ll have the willpower to handle the necessary reading. I’m remarkably lazy for someone who wants to know everything 😀

(Aside: holy crap, did I ALSO miss a fucking Nature paper about calcisponges’ honest to god ParaHox genes? Oh my god, oh my GOD!!! *sigh* This is also a piece of incredibly exciting information I’ve known for years, and I miss it when it actually comes out in a journal bloody everyone reads. You can tell I’ve been off-planet!)

References:

Chipman AD et al. (2014) The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology 12:e1002005

GarcĂ­a-Bellido DC et al. (2014) A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC Evolutionary Biology 14:214

Lauri A et al. (2014) Development of the annelid axochord: insights into notochord evolution. Science 345:1365-1368

Sansom RS et al. (2013) Atlas of vertebrate decay: a visual and taphonomic guide to fossil interpretation. Palaeontology 56:457-474

OH MY GOD A FILTER-FEEDING ANOMALOCARIDID!!!

ETA: OK, technically it should be “suspension-feeding”, because there’s a good chance that its feeding mechanics involved more than simple filtering (see comments). I hate retconning, so I’ll leave the post as it is aside from this addendum. Thanks for the heads-up, Dave Bapst 🙂

This is when I put everything resembling work aside to squee madly over a fossil.

(Imagine me grinning like crazy and probably bouncing up and down a bit in my seat)

Tamisiocaris is a newly “updated” beast from the Cambrian, and the coolest thing I’ve seen since that helicoplacoid on a stalk (most cool things come from the Cambrian, right?). It is the Cambrian equivalent of a baleen whale.

Anomalocaridids were close relatives of arthropods and are among the most iconic creatures of the Cambrian. Most anomalocaridids we know of were large, swimming predators with large head appendages bearing sturdy spines to grab prey and bring it to that trilobite-crunching pineapple slice mouth. Going with the whale analogy, they were more like the killer whales of their time (although they would be easy snacks for an actual killer whale). In fact, when the putative head appendage of Tamisiocaris was originally described by Daley and Peel (2010), the only odd thing they noted about it was that it was not hardened or obviously segmented the way those of Anomalocaris were.*

Tamisiocaris was already cool back then, because it was the first animal of its kind found at Sirius Passet in Northern Greenland, one of the lesser known treasure troves of fabulous Cambrian fossils. However, since then, more appendages have been found, and it turns out that those long spines had been hiding a fascinating secret.

They were… kind of hairy.

Vinther_etal2014-sf3_crop

Closer examination of the appendages shows that their long, slender spines bore closely spaced bristles, making each spine look like a fine comb (whole appendage and close-up of a spine above from Vinther et al. [2014]). With all the spines next to each other, the bristles would have formed a fine mesh suitable for catching prey smaller than a millimetre. Compared with modern filter-feeding animals, Tamisiocaris fits right in – it would have “fished” in a similar size range as a greater flamingo. Vinther et al. (2014) suggest that Tamisiocaris would have brought its appendages to its mouth (which isn’t among the known fossils) one at a time to suck all the yummies off.

These guys are tremendously interesting for more than one reason, as the new study points out. First, HOLY SHIT FILTER FEEDING ANOMALOCARIDIDS! (Sorry. I’m kind of excited about this.) Second, the mere existence of large**  filter feeders implies a richness of plankton people hadn’t thought existed at the time. Third, there is some remarkable convergent evolution going on here.

Often, really big plankton eaters evolve from really big predators – see baleen whales, basking sharks, and these humongous fish for example. It’s not an already filter-feeding animal growing bigger and bigger, it’s an already big animal taking up filter-feeding. The interrelationships of anomalocaridids suggest the same story played out among them – ferocious hunters begetting “gentle giants” in a group with a totally different body plan from big vertebrates. For all the dazzling variety evolution can produce, sometimes, it really rhymes.

And finally, Vinther et al. did something really cool that tickles my geeky side in a most pleasant way. In their phylogenetic analysis that they did to find out where in anomalocaridid evolution this plankton-eating habit came along, they found that Tamisiocaris was closely related to another anomalocaridid with (on a second look) not dissimilar appendages. They named the group formed by the two the cetiocarids – after an imaginary filter-feeding anomalocaridid created by artist John Meszaros for the awesome All Your Yesterdays project.

Man. That’s definitely worth some squee.

***

*Disclaimer: I’m basing this on the abstract only, since palaeontological journals are one of the unfortunate holes in my university library’s otherwise extensive subscriptions.

**For Cambrian values of “large” – based on the size of the appendages, this creature would have been something like two feet long.

***

References:

Daley AC & Peel JS (2010) A possible anomalocaridid from the Cambrian Sirius Passet Lagerstätte, North Greenland. Journal of Palaeontology 84:352-355

Vinther J et al. (2014) A suspension-feeding anomalocarid from the Early Cambrian. Nature 507:496-499

A bunch of cool things

From the weeks during which I failed to check my RSS reader…

1. The coolest ribozyme ever. (In more than one sense.)

I’ve made no secret of my fandom of the RNA world hypothesis, according to which early life forms used RNA both as genetic material and as enzymes, before DNA took over the former role and proteins (mostly) took over the latter. RNA is truly an amazing molecule, capable of doing all kinds of stuff that we traditionally imagined as the job of proteins. However, coaxing it into carrying out the most important function of a primordial RNA genome – copying itself – has proven pretty difficult.

To my knowledge, the previous record holder in the field of RNA copying ribozymes (Wochner et al., 2011) ran out of steam after making RNA strands only half of its own length. (Which is still really impressive compared to its predecessors!) In a recent study, the same team turned to an alternative RNA world hypothesis for inspiration. According to the “icy RNA world” scenario, pockets of cold liquid in ice could have helped stabilise the otherwise pretty easily degraded RNA as well as concentrate and isolate it in a weird inorganic precursor to cells.

Using experimental evolution in an icy setting, they found a variation related to the aforementioned ribozyme that was much quicker and generally much better at copying RNA than its ancestors. Engineering a few previously known performance-enhancing mutations into this molecule finally gave a ribozyme that could copy an RNA molecule longer than itself! It still wouldn’t be able to self-replicate, since this particular guy can only copy sequences with certain properties it doesn’t have itself, but we’ve got the necessary endurance now. Only two words can properly describe how amazing that is. Holy. Shit. :-O

*

Attwater J et al. (2013) In-ice evolution of RNA polymerase ribozyme activity. Nature Chemistry, published online 20/10/2013, doi: 10.1038/nchem.1781

Wochner A et al. (2011) Ribozyme-catalyzed transcription of an active ribozyme. Science 332:209-212

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2. Cambrian explosion: evolution on steroids.

This one’s for those people who say there is nothing special about evolution during the Cambrian – and also for those who say it was too special. (Creationists, I’m looking at you.) It is also very much for me, because Cambrian! (How did I not spot this paper before? Theoretically, it came out before I stopped checking RSS…)

Lee et al. (2013) used phylogenetic trees of living arthropods to estimate how fast they evolved at different points in their history. They looked at both morphology and genomes, because the two can behave very differently. It’s basically a molecular clock study, and I’m still not sure I trust molecular clocks, but let’s just see what it says and leave lengthy ruminations about its validity to my dark and lonely hours 🙂

They used living arthropods because, obviously, you can’t look at genome evolution in fossils, but the timing of branching events in the tree was calibrated with fossils. With several different methods, they inferred evolutionary trees telling them how much change probably happened during different periods in arthropod history. They tweaked things like the estimated time of origin of arthropods, or details of the phylogeny, but always got similar results.

On average, arthropod genomes, development and anatomy evolved several times faster during the Cambrian than at any later point in time. Including the aftermath of the biggest mass extinctions. Mind you, not faster than modern animals can evolve under strong selection – they just kept up those rates for longer, and everyone did it.

(I’m jumping up and down a little, and at the same time I feel like there must be something wrong with this study, the damned thing is too good to be true. And I’d still prefer to see evolutionary rates measured on actual fossils, but there’s no way on earth the fossil record of any animal group is going to be good enough for that sort of thing. Conflicted much?)

*

Lee MSY et al. (2013) Rates of phenotypic and genomic evolution during the Cambrian explosion. Current Biology 23:1889-1895

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3. Chitons to sausages

Aplacophorans are probably not what you think of when someone mentions molluscs. They are worm-like and shell-less, although they do have tiny mineralised scales or spines. Although they look like one might imagine an ancestral mollusc before the invention of shells, transitional fossils and molecular phylogenies have linked them to chitons, which have a more conventional “sluggy” body plan with a wide foot suitable for crawling and an armoured back with seven shell plates.

Scherholz et al. (2013) compared the musculature of a living aplacophoran to that of a chiton and found it to support the idea that aplacophorans are simplified from a chiton-like ancestor rather than simple from the start. As adults, aplacophorans and chitons are very different – chitons have a much more complex set of muscles that includes muscles associated with their shell plates. However, the missing muscles appear to be present in baby aplacophorans, who only lose them when they metamorphose. (As a caveat, this study only focused on one group of aplacophorans, and it’s not entirely certain whether the two main groups of these creatures should even be together.)

*

Scherholz M et al. (2013) Aplacophoran molluscs evolved from ancestors with polyplacophoran-like features. Current Biology in press, available online 17/10/2013, doi: 10.1016/j.cub.2013.08.056

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4. Does adaptation constrain mammalian spines?

Mammals are pretty rigid when it comes to the differentiation of the vertebral column. We nearly all have seven neck vertebrae, for example. This kind of conservatism is surprising when you look at other vertebrates – which include not only fairly moderate groups like birds with their variable necks, but also extremists like snakes with their lack of legs and practically body-long ribcages. Mammalian necks are evolutionarily constrained, and have been that way for a long time.

Emily Buchholz proposes an interesting explanation with links to previous hypotheses. Mammals not only differ from other vertebrates in the less variable numbers of vertebrae in various body regions; these regions are also more differentiated. For example, mammals are the only vertebrates that lack ribs in the lower back. In Buchholz’s view, this kind of increased differentiation contributes to adaptation but costs flexibility.

Her favourite example is the muscular diaphragm unique to mammals. This helps mammals breathe while they move, and also makes breathing more powerful, which is nice for active, warm-blooded creatures that use a lot of oxygen. However, it also puts constraints on further changes. Importantly, Buccholz argues that these constraints don’t all have to work in the same way.

For example, the constraint on the neck may arise because muscle cells in the diaphragm come from the same place as muscle cells associated with specific neck vertebrae. Moving the forelimbs relative to the spine, i.e. changing the number of neck vertebrae, would mess up their migration to the right place, and we’d end up with equally messed up diaphragms.

A second possible constraint has less to do with developmental mishaps and more to do with plain old functionality. If you moved the pelvis forward, you may not screw with the development of other bits, but you’d squeeze the space behind the diaphragm, which you kind of need for your guts, especially when you’re breathing in using your lovely diaphragm.

*

Buccholz E (2013) Crossing the frontier: a hypothesis for the origins of meristic constraint in mammalian axial patterning. Zoology in press, available online 28/10/2013, doi: 10.1016/j.zool.2013.09.001

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And… I think that approximately covers today’s squee moments 🙂

Oxygen and predators and Cambrian awesomeness (with worms!)

I didn’t plan to write anything today, but damn, Cambrian explosion. And polychaetes. I can’t not. Plus I’m going on holiday soon, so I might as well get something in before I potentially disappear off the internet. (Below: a Cambrian polychaete, Canadia spinosa, via the Smithsonian’s Burgess Shale pages.)

First, a confession.

I’m a bit of a coward about the Cambrian explosion.

Make no mistake, I love it. It’s fascinated me ever since I came across the heavily Stephen Jay Gould-flavoured account in The Book of Life. It’s an event that made the world into what it is today, with its complex ecosystems full of animals eating, cooperating or competing with each other. And it’s one of the great mysteries of palaeontology. What actually happened? What caused it? Why did it happen when it did? Why didn’t it happen again when animal life was nearly wiped out at the end of the Permian?

The problem is, I love it so much that I’m afraid to have an opinion about it. You have no idea how many times I wanted to discuss the big questions, only to shy away for fear of getting it wrong. Which is really kinda stupid, because no one has the one and only correct answer. Whether I’m qualified to comment on it is a different issue, but it wouldn’t be the first subject I comment on that I don’t fully understand.

So, here I take a deep breath and plunge into Sperling et al. (2013).

The abstract started by scaring me. It begins, “The Proterozoic-Cambrian transition records the appearance of essentially all animal body plans (phyla), yet to date no single hypothesis adequately explains both the timing of the event…” To which my immediate reaction was “why the fuck would you want a single hypothesis to explain it?” But luckily, they don’t. They actually argue for a combination of two hypotheses, which they think are more connected than we thought.

But let’s just briefly establish what the Cambrian explosion is.

I want to make this absolutely clear: it’s not the sudden appearance of modern animals out ot nowhere. It could be more accurately described as the appearance of basic body plans we traditionally classify as phyla, such as echinoderms, molluscs, or arthropods, in a relatively short geological period.

Doug Erwin (2011) trawled databases and literature to draw up a timeline of first appearances for animal phyla, and he found that they increase in number gradually over a period of 80 million years (see Erwin’s plot below).

Erwin2011-cumulativePhyla

Appearance of “phyla” also doesn’t equal appearance of modern animals, as Graham Budd has been known to emphasise (e.g. Budd and Jensen, 2000). For example, I already mentioned how none of the mid-Cambrian echinoderms recently described by Smith et al. (2013) would look familiar today. In fact, the modern classes of echinoderms, which include sea urchins, starfish and sea lilies, didn’t appear until after the Cambrian. Likewise, while there were chordates (our own phylum), and probably even vertebrates, in the Cambrian, such important vertebrate features as jaws or paired appendages were yet to be invented. (If memory serves, both of those inventions date to the Silurian.)

There is also a discussion to be had about the meaning and validity of concepts like a phylum or a body plan, but let’s not complicate things here. I have a paper to get to! 🙂

With that out of the way…

There is no doubt that something significant happened shortly before and during the Cambrian. Before the very latest Precambrian, fossils show little evidence of movement, of predation, or of the diverse hard parts that animals use to protect themselves or eat others today. All of these become commonplace during the Cambrian, establishing essentially modern ecosystems (Dunne et al., 2008).

There are many explanations proposed to account for the revolution. I’ve not the space (or the courage) to discuss them in any detail. If you’re interested, IIRC Marshall (2006) is a very nice and balanced review. (Link leads to a free copy.) However, we can discuss what Sperling et al. have to say about two of them.

The first hypothesis is oxygen, which likely became more abundant in the ocean towards the end of the Precambrian. That  could explain the timing, but maybe not the nature of the explosion. Oxygen levels impose a limit on the maximum size of animals, but what compels larger animals to “invent” more disparate body plans? (Also, on a side note, many Ediacaran organisms weren’t exactly tiny, so I’m not sure how much of a size limit there is in the first place.)

The second one is animal-on-animal predation (Sperling et al. prefer the term “carnivory”), which can lead to predator-prey arms races and therefore encourage the evolution of innovations like shells or burrowing or jaws that give one party an edge. This is a decent enough basis for body plan innovation, but it applies for any time and place with animals. So if carnivory is the explanation, why did the explosion happen when it did?

Because, Sperling et al. argue, carnivory and oxygen are linked.

I’m intrigued by their approach. They’re not looking at fossils in this study at all. (I always like it when palaeontology and the biology of the living join forces!) They are looking at oxygen-poor habitats in modern oceans. Specifically, they asked how low oxygen levels affect polychaete worm communities.

Why polychaetes? The authors give a list of reasons. One, polychaetes are really, really abundant on the seafloor, and particularly so in low-oxygen settings. Two, different species feed in almost every conceivable way from filtering plankton through chewing through sediment to flat out devouring other animals, and their feeding mode can usually be guessed even if you haven’t seen that particular species eat. Three, they are actually quite good at handling oxygen limitation. This is important because back in the Precambrian, all animals would have been well adapted to a low-oxygen environment, so a group that can tolerate the same may be the best comparison. (They do note that  a previous study of a single low-oxygen site that took other animals into account came up with similar results to theirs.)

They worked partly with pre-existing datasets that met a set of criteria designed to get a complete and unbiased view of local polychaete diversity. In total, they analysed data from 68 sites together featuring nearly a thousand species of worms. They also had some of their own data.

They categorised their study sites into four levels of oxygen deprivation, and counted numbers of carnivorous individuals and species at each site. They came to the conclusion that lack of oxygen basically makes carnivores disappear. The lowest-oxygen samples contained fewer carnivores on both the individual and species levels, and they were more likely to be devoid of predators altogether (# species plot from the paper below):

sperling_etal2013-fig2c

There are a couple of different ways in which lack of oxygen could limit predators. For example, the aforementioned size limit is one, because it’s good for a predator to be larger and stronger than its prey. But the biggest factor according to the authors is the energy required for an active predatory lifestyle. While a suspension feeder can sit in one place all day and only move to stuff a food-laden tentacle into its mouth, a predator has to find, subdue and eat its prey, which are all pretty expensive activities. Then it also has to digest a sudden, large meal, whereas the suspension feeder’s digestion works at a low and steady rate. Animals can get energy from a variety of metabolic processes, but by far the most efficient route requires oxygen. And that really sucks when you are a hunter who might need large amounts of energy at short notice.

Hmm…

Although I’m quite intrigued by the study, there are a couple of issues that bother me. For example, as far as I could tell, all of the study sites included in the analyses were low on oxygen. I would have liked to see them compared to “normal” sites, in particular because the trend in predator abundance wasn’t a neat straight upwards line. In fact, the least oxygen-deprived habitats appeared less predator-infested than slightly more oxygen-poor ones. What’s going on there?

In terms of interpretation in relation to the Cambrian, I also would have liked to see a comparison of the oxygen levels at their study sites to what’s estimated for the geological periods in question. I take it they just didn’t have precise enough estimates, because one of the things they discuss in the closing paragraph is the need to measure just how much oxygen went into the oceans during this late Precambrian oxygen increase.

And my semi-silly question is, how does this apply to “predators” who don’t run around chasing after and wrestling with prey? For example, sea anemones might be perfectly happy to eat large creatures. But they don’t really do much. They just sit and wait, and if a poor fish stumbles onto their sticky venomous tentacles, tough luck for it. Or there’s the unknown predator that drilled holes in late Precambrian Cloudina specimens (Bengtson and Zhao, 1992). Cloudina was sessile, the creature didn’t have to chase it… Predators such as these still have to cope with the energy demands of digesting sudden large meals, I suppose, so maybe the energetics idea still applies. And of course, if there’s no oxygen, large prey is less likely to be swimming around bumping into your tentacles.

Is this “the” explanation of the Cambrian explosion? Probably not, says the cynic in me. I highly doubt we’re done with that question. Is it a good explanation? Well, it is certainly evidence-based, and I like it that it tries to take different factors together and in context. What I don’t think it does is explain the uniqueness of the Cambrian. A thousand words or so ago, I mentioned the Permian extinction. That cataclysm very nearly left the earth devoid of animals. Afterwards, there was certainly enough oxygen for predators to thrive in the sea, and indeed they did, from sea urchins to ichthyosaurs. So why didn’t the first 40 million years of the Mesozoic era beget many new phyla the way the first 40 million years of the Palaeozoic did? Is that just an artefact of our classifications or was something really fundamentally different going on?

I ain’t Jon Snow, but when it comes to the Cambrian, I still feel like I know nothing…

***

References:

Bengtson S & Zhao Y (1992) Predatorial borings in Late Precambrian mineralized exoskeletons. Science 257:367-369

Budd GE & Jensen S (2000) A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews 75:253-295

Dunne JA et al. (2008) Compilation and network analyses of cambrian food webs. PLoS Biology 6:e102

Erwin DH (2011) Evolutionary uniformitarianism. Developmental Biology 357:27-34

Marshall CR (2006) Explaining the Cambrian “explosion” of animals. Annual Review of Earth and Planetary Sciences 34:355-384

Smith AB et al. (2013) The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid. Nature Communications 4:1385

Sperling EA et al. (2013) Oxygen, ecology, and the Cambrian radiation of animals. PNAS 110:13446-13451

I couldn’t resist

Damn, I said I wasn’t going to talk about the Moroccan helicoplacoid-on-stalk, but it’s just so. Bloody. Amazing.

Here it is in its full glory, from the supplementary figures of Smith and Zamora (2013). Left is a cast of a young specimen, right is the authors’ reconstruction of the adult creature:

helicocystis_casthelicocystis_recon

So… the thing is a transitional form all right. It’s got a little stalk and cup like eocrinoids, built with a rather irregular arrangement of mineralised plates. On top of that it has a spiral body like helicoplacoids. It has ambulacra, the “rays” with porous plates where the tube feet that characterise living echinoderms can come out. This photo of the underside of a starfish is a pretty nice illustration of ambulacra (the white regions with little holes) and tube feet:

Even more interestingly, the new beastie (christened Helicocystis moroccoensis by the authors) seems to have five of them, like modern echinoderms (and a lot of extinct types, including eocrinoids). Helicoplacoids do have ambulacra, but only three or a single Y-shaped one, depending on interpetation.

Again unlike (one interpretation of) helicoplacoids but like modern echinoderms, the mouth of Helicocystis is right at the stalkless end. It’s also surrounded by an arrangement of skeletal plates that resembles more “conventional” echinoderms and has no equivalent in helicoplacoids proper. It’s about as neat a transitional form as you could hope for.

The question is which way the transition goes. It could be that the familiar five-rayed echinoderms are derived from a helicoplacoid-like ancestor, going through something like this guy. Or it could be that helicoplacoids are actually weird even for echinoderms, and their ancestors were more conventional stalked, five-armed beasties that lost their proper echinoderm shapes via something like Helicocystis.

Smith and Zamora actually did a phylogenetic analysis, but it’s not that helpful IMO. The tree in the paper is very pretty, and it says Helicocystis is the next branch after helicoplacoids on the path leading to “proper” echinoderms. The tree in the supplementary figures actually has measures of statistical support on it – which pretty confidently put Helicoplacus, Helicocystis, and a bunch of less weird echinoderms, together.

However, the relationships within that group are, shall we say, a little bit fluid. Granted, I come from a more sequency background and don’t often have to deal with morphology-based trees or parsimony as the method of analysis – but I’d definitely view a 56% bootstrap support with a big dose of scepticism, and this is the number they got for the hypothesis that Helicocystis is more closely related to “proper” echinoderms than to Helicoplacus. The other measure they display doesn’t make me any more confident about the relationship.

(I find it kind of amazing they got any resolution at all in that tree – with only 17 characters, some of which aren’t applicable to all species, and only nine species to begin with… yeah. The whole phylogenetic analysis is far from ideal even if it’s the best they could think of.)

So, based on that tree, the phylogenetic hypothesis they present is, at this point, just a plausible hypothesis. That doesn’t lessen the value of Helicocystis, though. The creature is still a damn neat transitional form – we just can’t be terribly sure which way the transition went.

There’s some interesting speculation in the paper about developmental evolution (yay!). Smith and Zamora point out that the spirally bit in Helicocystis looks like a complete helicoplacoid; the stalk and cup are kind of tacked onto that. The tissues of most modern echinoderm adults come from two different places: regular old tissues of the larva, and a special set of cells set aside for adult-making purposes*. So Smith and Zamora hypothesise that the two-part body of Helicocystis marks the point where this dual origin appeared. (Or, if they’re wrong about the phylogeny, the point where proto-helicoplacoids lost it?)

There’s also another interesting bit of evo-devo speculation (mixed with a bit of “eco”) about the stalk. Full-grown Helicocystis have pretty small stalks compared both to their own young and more typical stalked echinoderms. The authors wonder if this is because stalks for attachment originally functioned to help young echinoderms settle in a comfortable place, and only later became important for adults. I’m not sure how much sense that actually makes, and of course we only have a single species of Helicocystis to go by, but hey, ideas are fun.

Helicocystis has a random weird quirk as well, in that its spirals curl the opposite way to every proper helicoplacoid. That sort of variation happens even within species (e.g. in snail shells), but isn’t it a weird coincidence that such a unique creature should also twist the wrong way?

One thing is for sure: this beast is made of pure, distilled awesome. I think we should make a new Archaeopteryx out of it. Invertebrates need their evolutionary icons, too!

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*And that’s a nice reminder for me, because I thought they basically threw away the larva. Apparently I need a refresher on echinoderm development. Or just a reminder that not all echinoderms are sea urchins. The funny thing is a couple of years ago I actually specifically read and puzzled over literature discussing what comes from where in various echinderms…

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Reference:

Smith AB, Zamora S (2013) Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B advance online publication 26/06/2013, doi:10.1098/rspb.2013.1197

Petrified strawberries and the cnidarian that isn’t

In the last few weeks, tons of squee-worthy stuff has accumulated on my backlog. The echinoderm transitional form from Cambrian Morocco I got so excited about is now officially described (Smith and Zamora, 2013), Dennis Duboule and his team put out some really cool findings about how vertebrate Hox clusters work that connect to my old fascination with limb evo-devo (Andrey et al., 2013), the developmental hourglass returned (Schep and Adryan, 2013)…

I kind of regret not gushing about all of them, but let’s face it, I’m not gonna ever do that. Looking over the things I bookmarked recently, I decided I’d rather not ignore Yasui et al. (2013), though. One, it’s about early animals, two, it exploits one of the greatest treasures the fossil record has to offer: the record of ancient development. I almost don’t care what the findings are, because the fact that we can follow a 530+-million-year-old creature from egg to adult is so staggeringly awesome in itself that everything else pales in comparison.

The paper looks at a tiny creature known from the earliest Cambrian of China. The beastie is called Punctatus and looks something like this (the authors’ interpretation of its development from fig. 4 of the paper):

The observations in this paper come from some 10 thousand specimens of various developmental stages from a couple of different Punctatus species. With such an abundance of fossils covering the animal’s life cycle, it is possible to connect the different stages and identify them as the same animal. So how did Punctatus develop and what kind of animal was it?

The earliest development took place inside a smooth egg membrane. Broken or CT-scanned embryos show that the creatures went through a nice blastula stage that looked like a simple, hollow ball of cells which were maybe a bit fatter on one side than the other (below, left). This type of blastula is quite common, found in animals as disparate as jellyfish (middle, from celldynamics.org) and sea urchins (right, from exploratorium). So the blastulae don’t tell you much about the affinities of the creature. In fact, while the authors use the leftmost embryo as a pretty illustration, they’re not even sure this particular specimen belongs to Punctatus. (Which is not really surprising.)

coeloblastulae

By the time young Punctatus hatch from their eggs, they are much more identifiable. The authors compare them to strawberries (awwww! ^.^). They are spiny all over, slightly pointy on one end and slightly flattened on the other, and the flattened end is divided into five parts by a star-like pattern of Y-shaped grooves. At the centre of the star, there is the blastopore, the opening of the embryonic gut, which seems to develop straight into the mouth in this creature. Punctatus never develops another gut opening. The simple blastopore = mouth equation again isn’t terribly informative, since a lot of animals follow it, and it’s the most straightforward way to make a mouth. The only thing the lack of a through gut tells us is something that was already fairly obvious – Punctatus is not a bilaterian.

The early stages also exclude another group from the list of possible identities, that is ctenophores. Early embryos of modern ctenophores (comb jellies/sea gooseberries) have very unequal-sized cells (see image at the top of this article), and no such embryos are known from the deposits preserving Punctatus specimens. (Although given what I recently learned about living ctenophores having a very recent common ancestor, I wouldn’t bet on what their Cambrian ancestors were up to…)

Thirdly, embryos that haven’t yet hatched also tell us something important about the adults. The prickly covering of these animals had apparently been interpreted as the remnants of a tube in which the animal proper lived – but this covering clearly appears before the baby even pops out of the egg, and the mouth forms right in the middle of it. All of that makes it more likely to be the animal’s skin. And that weakens a possible link to a group of extinct tube-dwelling animals that are much more plausibly related to jellyfish.

After hatching, development enters a new stage. In young and adult Punctatus specimens, the strawberry-like hatchling body remains in place, but a new body region appears at the mouth end, which has a ringed appearance and no spines. Presumably, individuals with more rings were older.

CT cross-sections of such specimens (C-E below) show a huge, empty body cavity, with a small sac-like gut attached to the mouth. There’s apparently no “stuff” between the gut and the body wall: no mesenteries anchoring the gut, no jelly or mass of cells filling in the body cavity, just big fat nothing. This is unlike not just bilaterians or ctenophores, but also cnidarians, in which the gut wall tends to be much closer to the body wall, and a jelly-like layer containing a varying amount of cells fills any gaps between the two.

From this point, the basic body plan doesn’t seem to change. Specimens with only a couple of rings and those with a dozen have the same small gut and large body cavity. There’s nothing we might call metamorphosis – unlike most cnidarians, Punctatus didn’t have a larval stage. (BTW, can someone tell me what the hell the lumpy bit on top of G above is? The paper doesn’t bother to explain as far as I could tell, and it bugs me.)

An intriguing (and rather pretty) part of the animal is the mouth end, what the authors call the “oral ruffle”. You’ll see why it’s called that if you look at figure 3:

This is an inferred developmental series of the mouth region. The five-pointed star of the hatchlings develops into ten finely striped folds emerging from the body surface, and as the animal grows, a new oral ruffle appears inside the previous one. The old ruffle then becomes part of the body wall, forming the next ring. Rinse and repeat. There are no tentacles at any point, although this might still turn out to be an artefact of preservation.

Tenfold symmetry, stacks old oral ruffles, no tentacles, building an adult body on top of an intact piece of embryo – the whole thing is quite unlike your typical cnidarian. Or, indeed, your typical anything else. The authors use the unusual developmental and body plan features of this creature to question its previous assignment to cnidarians, but beyond that, they are unsure what to make of it.

Well, this is the Early Cambrian, when a lot of now-extinct animal lineages were kicking around. Of course they would give us classification headaches! 😉 Which probably means that we know an awful lot about the development of a member of a long-extinct lineage. That’s a comparative embryology goldmine right there, folks!

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References:

Andrey G et al. (2013) A switch between topological domains underlies HoxD genes colinearity in mouse limbs. Science 340:1234167, doi:10.1126/science.1234167

Schep AN, Adryan B (2013) A comparative analysis of transcription factor expression during metazoan embryonic development. PLoS ONE 8:e66826

Smith AB, Zamora S (2013) Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B advance online publication 26/06/2013, doi:10.1098/rspb.2013.1197

Yasui K et al. (2013) A diploblastic radiate animal at the dawn of cambrian diversification with a simple body plan: distinct from Cnidaria? PLoS ONE 8: e65890

Lotsa news

Hah, I open my Google Reader (damn you, Google, why do you have to kill it??? >_<), expecting to find maybe a handful of new articles since my last login, and instead getting both Nature and Science in one big heap of awesome. The latest from the Big Two are quite a treat!

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By now, of course, the internet is abuzz with the news of all those four-winged birdies from China (Zheng et al., 2013). I’m a sucker for anything with feathers anywhere, plus these guys are telling us in no uncertain terms that four-wingedness is not just some weird dromaeosaur/troodontid quirk but an important stage in bird evolution. Super-cool.

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Then there is that Cambrian acorn worm from the good old Burgess Shale (Caron et al., 2013). It’s described to be like modern acorn worms in most respects, except it apparently lived in a tube. Living in tubes is something that pterobranchs, a poorly known group related to acorn worms do today. The Burgess Shale fossils (along with previous molecular data) suggest that pterobranchs, which are tiny, tentacled creatures living in colonies, are descendants rather than cousins of the larger, tentacle-less and solitary acorn worms. This has all kinds of implications for all kinds of common ancestors…

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Third, a group used a protein from silica-based sponge skeletons to create unusually bendy calcareous rods (Natalio et al., 2013). Calcite, the mineral that makes up limestone, is not normally known for its flexibility, but the sponge protein helps tiny crystals of it assemble into a structure that bends rather than breaks. Biominerals would just be ordinary rocks without the organic stuff in them, and this is a beautiful demonstration of what those organic molecules are capable of!

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And finally, Japanese biologists think they know where the extra wings of ancient insects went (Ohde et al., 2013). Today, most winged insects have two pairs of wings, one pair on the second thoracic segment and another on the third. But closer to their origin, they had wing-like outgrowths all the way down the thorax and abdomen. Ohde et al. propose that these wing homologues didn’t just disappear – they were instead modified into other structures. Their screwing with Hox gene activity in mealworm beetles transformed some of the parts on normally wingless segments into somewhat messed up wings. What’s more, the normal development of the same bits resembles that of wings and relies on some of the same master genes. It’s a lot like bithorax mutant flies with four wings (normal flies only have two, the hindwings being replaced by balancing organs), except no modern insect has wings where these victims of genetic wizardry grew them. The team encourage people to start looking for remnants of lost wings in other insects…

Lots of insteresting stuff today! And we got more Hox genes, yayyyy!

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References:

Caron J-B et al. (2013) Tubicolous enteropneusts from the Cambrian period. Nature advance online publication 13/03/2013, doi: 10.1038/nature12017

Natalio F et al. (2013) Flexible minerals: self-assembled calcite spicules with extreme bending strength. Science 339:1298-1302

Ohde T et al. (2013) Insect morphological diversification through the modification of wing serial homologs. Science Express, published online 14/03/2013, doi: 10.1126/science.1234219

Zheng X et al. (2013) Hind wings in basal birds and the evolution of leg feathers. Science 339:1309-1312

Echinoderm bonanza

Smith et al. (2013) has been sitting on my desktop waiting to be read for the last month or so. Man, am I glad that I finally opened the thing. I’m quite fond of echinoderms, and this paper is full of them. Of course. It’s about echinoderms. Specifically, it’s about the diverse menagerie of them that existed, it seems, a little bit earlier than thought.

The brief little paper introduces new echinoderm finds from two Mid-Cambrian formations in Morocco, which at the time was part of the great continent of Gondwana. As far as I’m concerned, it was worth reading just for this lineup of Cambrian echinoderms. I mean, echinoderms are so amazingly weird in such a variety of ways. They’re a delight.

Smith_etal2013-fig3_decorated

(The drawings themselves are from Fig. 3. of the paper; I rearranged them to fit into my post width, and the boxes are my additions. Dark box = new groups/species from Morocco, light grey box = known groups/species whose first appearance was pushed back in time by the Moroccan finds.)

Although none of the creatures above belong to the living classes of echinoderms, they display a wide range of body plans. You could say their body plans are more diverse* than those of living echinoderms. (And if you said that, the ghost of Stephen Jay Gould would nod approvingly.) For example, modern echinoderms tend to have either (usually five-part) radial symmetry (any old starfish) or bilateral symmetry that clearly comes from radial symmetry (heart urchins).

In these Early- to Mid-Cambrian varieties, you can see some five-rayed creatures, some that are more or less bilateral without any obvious connection to the prototypical five-point star, animals that are just kind of asymmetric, and those strange spindle-shaped helicoplacoids that look like someone took an animal with radial symmetry and wrung it out. And then there are all the various arrangements of arms and stalks and armour plates that I tend to gloss over when reading about the beasts. (Yeah. I have no attention span.)

The Morroccan finds have some very interesting highlights. The second creature in the lineup above is one of them. Its top half looks like a helicoplacoid such as Helicoplacus itself (first drawing). It’s got that characteristic spiral arrangement of plates and a mouth at the top end. However, unlike previously known helicoplacoids, it sits on a stalk that resembles the radially-symmetric eocrinoids (like the creature on its right). It’s a transitional form all right, though we’ll have to wait for future publications and perhaps future discoveries to see which way evolution actually went. It’ll already help palaeontologists make sense of helicoplacoids themselves, though, which I gather is a big thing in itself. The authors promise to publish a proper description of the creature, which is really exciting.

The other exciting thing about the Moroccan echinoderms is their age. As I already hinted at with my grey boxes, the new fossils push back the known time range of many echinoderm body plans by millions of years. This means that the wide variety of body plans we saw above was already present as little as 10-15 million years after the first appearance of scattered bits of echinoderm skeleton in the fossil record.

Smith et al. argue that this is a fairly solid conclusion based on the mineralogy of echinoderm skeletons. Organisms with calcium carbonate hard parts have a tendency to adopt the “easiest” mineralogy at the time they first evolve skeletons. Seawater composition changes over geological time; most importantly, the ratio of calcium to magnesium fluctuates. Calcium carbonate can adopt several different crystal forms, and the Ca/Mg ratio influences which of them are easier to make. So when there’s a lot of Mg in the sea, aragonite is the “natural” choice, whereas low Mg levels favour calcite.

The first appearance of echinoderms around 525 million years ago coincides with a shift in ocean chemistry from “aragonite seas” to “calcite seas”. Echinoderms and a bunch of other groups that first show up around that time have skeletons that are calcite in their structure but incorporate a lot of Mg. Since the ocean before was favourable to aragonite, it’s unlikely that echinoderm skeletons appeared much earlier than this date. In other words, echinoderm evolution during this geologically short period was truly worthy of the name “Cambrian explosion”.

That is, of course, if the appearance of echinoderm skeletons precedes the appearance of echinoderm body plans. The oldest of our Cambrian treasure troves of soft-bodied fossils, such as the rocks that yielded the Chengjiang biota of China, are roughly the same age as the first echinoderm skeletons. However, they don’t contain undisputed echinoderms as far as I can tell (Clausen et al., 2010). Proposed “echinoderms” from before the Cambrian are even less accepted. Of course, the unique structure of echinoderm skeletons is easy to recognise, but how do you identify an echinoderm ancestor without such a skeleton? (Is all that bodyplan diversity even possible without hard skeletal support?)

Caveats aside, this Moroccan stuff is awesome. And also, if my caveat proves overly cautious, echinoderms did some serious evolving in their first few million years on earth. A supersonic ride with Macroevolution Airlines?

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*OK, if I want to be absolutely pedantic, and I do, then body plans are disparate rather than diverse. “Disparity” in palaeontological/evo-devo parlance refers to how different two or more creatures are. Diversity means how many different creatures there are. Maybe I should do a post on that, actually.

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References:

Clausen S et al. (2010) The absence of echinoderms from the Lower Cambrian Chengjiang fauna of China: Palaeoecological and palaeogeographical implications. Palaeogeography, Palaeoclimatology, Palaeoecology 294:133-141

Smith AB et al. (2013) The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid. Nature Communications 4:1385

So… much… STUFF!

Gods, this is what I’m faced with all the time. Someone needs to tell me how proper science bloggers pick articles to discuss, because I just get my RSS alerts, start squeeing, and end up not writing about anything because damn, I WANT TO WRITE ABOUT EVERYTHING!

I give up. I’ll just dump all the cool stuff that’s accumulated on my desktop and bookmark bar here and return to lengthy meandering whenever I don’t feel like I’ve been caught in a bloody tornado 😉

So, here is some Cool Stuff…

(1) A group measured the rate of DNA decay in 158 moa bones of known age from three sites. Really cool stuff, to go out and directly measure how ancient DNA disappears from dead things under more or less identical conditions. The unsurprising result is that DNA decays exponentially, a bit like radioactive material. This suggests that the main cause of the decay is random breaking of the strands. The surprising bit is that this happens much more slowly than previously estimated, suggesting that in ideal (read: frozen) conditions, it might be worth looking for preserved DNA in samples as old as a million years.

(On a side note, if you ever get a chance to see a talk by Eske Willerslev, one of the authors and a leading expert on ancient DNA, don’t miss it. The man is absolutely hilarious.)

– Allentoft ME et al. (2012) The half-life of DNA in bone: measuring decay kinetics in 158 dated fossils. Proceedings of the Royal Society B FirstCite article, available online 10/10/2012, doi: 10.1098/rspb.2012.1745

(2) The beaks of the finches, or mixing and matching developmental recipes. This study examines the genetic basis of beak shape in three little birds closely related to Darwin’s famous finches. The three finches, just like Darwin’s, share the same basic beak shape, only bigger or smaller. However, there seem to be two distinct developmental programs at work, using different genes and parts of the skeleton to orchestrate beak development. One of the three newly investigated species (the one most closely related to Darwin’s finches) apparently uses the same developmental program as its more famous relatives, even though its beak is shaped more like the other two birds studied here. I told you – genetics, development and homology are complicated 😉

– Mallarino R et al. (2012) Closely related bird species demonstrate flexibility between beak morphology and underlying developmental programs. PNAS 109:16222–16227

(3) Armoured fossil links worm-like molluscs to chitons. There’s a little-known group (or groups) of molluscs called aplacophorans that have only a coat of tiny spicules instead of shells and look more like worms than “proper” molluscs. Exactly where they fit into our picture of mollusc evolution has been controversial to say the least – they could represent an old lineage separate from other molluscs, they could be related to cephalopods, they could be related to chitons, they could be one group or they could be two lineages in completely different places on the tree… Well, a new fossil named Kulindroplax seems to argue for the chiton connection: the animal has the characteristic armour plates of a chiton on an aplacophoran-like body. Similar creatures have been discovered before, but this guy with its detailed 3D preservation provides the clearest evidence of the link so far.

– Sutton MD et al. (2012) A Silurian armoured aplacophoran and implications for molluscan phylogeny. Nature 490:94-97

(4) More cool fossils – this time straight from my beloved Cambrian. Nereocaris, a newly described Burgess Shale arthropod, suggests to its discoverers that the earliest arthropods weren’t predators prowling the seafloor, but swimmers who might have been filter feeders and certainly weren’t predators. The animal has a bivalved shell around its front end, similar to many other Cambrian swimming arthropods, and a long abdomen with paddles at the end. It bears the arthropod hallmark of a hardened and jointed exoskeleton, but it lacks specialised limbs such as antennae or mouthparts. In a cladistic analysis of arthropods and their nearest relatives, the new species comes out on the first branch within true arthropods, and the next few branches as we move towards living arthropods all contain similar shelled, swimming creatures. Since the non-arthropods closest to the real thing (i.e. anomalocaridids) were also fin-tailed swimmers, this arrangement makes the transition between them and true arthropods smoother than previously thought. It also suggests that the hard exoskeleton so characteristic of arthropods originally functioned in swimming – perhaps as an anchor for swimming muscles.

– Legg DA et al. (2012) Cambrian bivalved arthropod reveals origin of arthrodization. Proceedings of the Royal Society B FirstCite article, available online 10/10/2012, doi: 10.1098/rspb.2012.1958

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And … there was also

… but it’s almost bedtime, and if I wanted to summarise every one of those, I’d be here all weekend 😦

See, this is why being a science nerd today is both amazing and frustrating. There’s just so. Much. Stuff.